For a beginner, which one would you guys recommend?I don't have much experience with landscapes, and I donīt know if an 18mm make that much difference to a 17mm.I'm also planning in going to France in December, so this lens would be used to take lots of photos around Paris.

Another question: features like IS make so much difference? I never had a lens without it.

I started with the Canon 17-40 as my wide lens, but found it was not wide enough for me. Its a good lens, but with the 50D, it has the same viewing angle as a 28mm on a full-frame 35mm. I had a 20mm on the Canon film 35mm camera, and 17mm just wasn't wide enough.

That being said, the Canon 10-22 is the equivalent of a 16mm on full-frame 35mm camera, which is very close to being in the fish-eye range. There is a bit of barrel distortion, but Lightroom fixes that very nicely. There is also some geometric distortion due to the wide angle of view. If this bothers you, you would not often use the lens at 10mm.

I am often quite amazed at the images the 10-22 produces, even at 10mm and I generally have it on the camera except when a long lens is required.

Since I print large canvasses of my landscape work I need lots of resolution. I've found I use my 70-200 mm Canon IS the most and stitch together 3 to 10 shots of my subject. What that does is gives me 1 GB+ file size after stitching for printing any size I need. But more important it eliminates the wide lens distortions that can ruin the impact of truly large landscape panoramas--big center mountain and puny mountains left and right. If I need to incorporate a foreground element or two I usually shoot them separately and add them back into the final image in post production.

I had the 10-22mm Canon lens for my 40D. It's a common wide-angle zoom lens, equivalent in angle of view to a 16-35mm lens on a full-frame camera like the 5D. Now, at the widest end it's really wide - MUCH wider than the 28mm you have as your widest focal length right now. But the range is actually pretty good for you -- you'd have 10-22 and 28-135 covered, with no overlap and not much missing in the middle. My 10-22 made very high quality images, though like most WA zooms it suffered from some softness in the corners.

A really wide lens like this requires some care in use. Just pointing it at a distant scene and zooming out to the widest setting results in unsatisfying images. Finding an interesting foreground and juxtaposing foreground/background can provide a great feeling of depth. (The attached photo was made at 10mm on a 40D.) It takes some effort and work to use well and get the best results.

Another lens that I could recommend (perhaps more strongly) is the Canon 15-85. This is an EF-S lens, so it's made for your 50D, and it covers a focal range from a solid wide angle (24mm equivalent) to a nice short telephoto (135mm equivalent.) It's a great general purpose and landscape lens, has image stabilization, and the widest focal length is a little more manageable than the 10mm. I use a 24-105/4 on my 5D, and this is the logical equivalent for the smaller-sensor cameras.

EDIT: Another option is the Sigma 12-24mm lens. It's a full-frame lens, so it can be used on your 50D, where it will be a very wide lens, but it can also be used on a full frame camera, where it is a really, really wide lens. I have one at work, and use it on both kinds of cameras. Image quality is excellent -- it's my architecture lens. For me it's best used on a tripod, at f/11, mirror locked up, carefully focused with live view (standard careful landscape and architecture technique.)

I don't know much about APS-C. I can imagine that's a old technology right? What's the tech nowadays cameras are coming with now?

APS-C is not a technology but only a name for a certain sensor size. In principle the larger the sensor the better in terms of overall image quality. Sensors the size of film (at least the type of film you are probably familiar with) are called full-frame or 35mm are very expensive to make because the require some extra steps in the production. APS-C sized sensors can be made cheaper and in addition have the advantage of allowing for smaller and lighter bodies and lenses(!).

If you don't have any experience with wide-angle lenses, why not rent one or two to figure out yourself what you want? People here are very helpful and will certainly not recommend any crap lenses but in the end they have to suit you and not anyone else.

APS-C is not a technology but only a name for a certain sensor size. In principle the larger the sensor the better in terms of overall image quality. Sensors the size of film (at least the type of film you are probably familiar with) are called full-frame or 35mm are very expensive to make because the require some extra steps in the production. APS-C sized sensors can be made cheaper and in addition have the advantage of allowing for smaller and lighter bodies and lenses(!).

If you don't have any experience with wide-angle lenses, why not rent one or two to figure out yourself what you want? People here are very helpful and will certainly not recommend any crap lenses but in the end they have to suit you and not anyone else.

Bottom line:1 - I wan't a lens that I can shoot landscapes and also use for shotting weeding and garduations.2 - For shooting weedings, I believe I need a bright lens (Aperture of f/2.8 or less (2.0, 1.8, etc.))3 - I wan't a lens that I can use in my Canon 50D, but that will also fit in a Fullframe (that I will buy next year).

Bottom line:1 - I wan't a lens that I can shoot landscapes and also use for shotting weeding and garduations.2 - For shooting weedings, I believe I need a bright lens (Aperture of f/2.8 or less (2.0, 1.8, etc.))3 - I wan't a lens that I can use in my Canon 50D, but that will also fit in a Fullframe (that I will buy next year).

Having that in mind, do you have any lens to recommend?

Not really. The Canon 16-35/2.8 will fit all three criteria, but it's not particularly wide on a 50D. I guess it's wider than what you have already, so that will help some. It would be a decent "normal zoom" on the 50D, so it would be very useful for weddings and it has a nice bright aperture, though no image stabilization. It will be very wide on a full frame camera.